1. Field of the Invention
The invention relates to a diagnostic device and method for monitoring the operation of a control loop of an automation system.
2. Background of the Invention
The maintenance and servicing of automation systems can be improved if subsystems or components are monitored to ensure that they are functioning correctly. In the event of reduced operational performance, it is possible to intervene selectively at the appropriate point of the system with maintenance, servicing or fault-clearance measures. Components of automation systems are typically interconnected in control loops. Identifying of reduced capacity of individual control loops to enable timely and selective intervention at the appropriate point of the system with measures for servicing or controller optimization would be enhanced by permanent and automatic monitoring of the control quality of control loops in the system.
A diagnostic method for monitoring the operation of a control loop is known from EP 1 528 447 B1. For a control loop having a substantially stationary status, i.e. with a substantially constant setpoint selection, the variance of a sequence of actual-value data is determined as a stochastic feature and evaluated for analysis of the status of the control loop. In the event of an excitation of the control loop corresponding to a setpoint step-change, the relative overshoot or the settling ratio, i.e. the quotient of rise and settling time of the control variable, can be evaluated as deterministic features for the analysis of the control loop status.
Automation systems, however, typically include numerous control loops to which known monitoring methods cannot be applied or can only be applied with extreme restrictions. This is in particular the case when, in the context of meshed control structures, controllers are controlled as slave controllers by another higher-ranking controller by means of setpoint selection. Hence, this applies to all slave controllers in cascade control systems and all slave controllers in ratio control systems. Since these regulators obtain their setpoints from another functional module of the interconnection in the meshed controller structure, the setpoint can change constantly and there are neither any temporal phases with a constant setpoint nor clean setpoint step-changes that can be evaluated in the context of the known monitoring of the control loop operation with reference to the determination of a stochastic feature or a deterministic feature. If the setpoint is obtained by a master controller in the context of a cascade control system, the setpoint fluctuates to the same degree as the manipulated variable of the master controller. If the setpoint is generated by a ratio module in the context of a ratio control system, the setpoint of the ratio controller is typically the actual value of another control loop, which can also be changed during operation, multiplied by a constant factor. Therefore, using the prior art method, subordinate control loops are generally excluded from monitoring.